Probe cards are used to electrical test semiconductor devices during fabrication. Probe cards are critical to ensuring quality of semiconductor devices and in maximizing yield of semiconductor fabrication processes. Accordingly, it is important to ensure that probe cards are in good working order.
Inspection of probe cards involves an assessment of the physical characteristics of each probe as well as the electrical characteristics of not only each probe, but of the entire probe card itself and the fixtures, known as motherboards, to which probe cards are coupled during use.
While probe cards can exert hundreds of pounds of force against a semiconductor wafer during the probing process, each individual probe in a probe card is very delicate and typically experiences no more than a few grams of force. Accordingly, it is necessary to take great care to avoid damaging probes during inspection. Probes are also very small, which can make it difficult to address individual probes for mechanical or electrical testing. One way that probes are addressed individually is by using a very small conductive post that has a flat top with a diameter that is only slightly larger than that of a probe tip. The flat top of the conductive post is addressed to each of a selected group of probes of a probe card (or all of them) to determine the electrical properties of the probe itself and the circuit to which the probe is connected. What is more, the conductive post may be biased or overtraveled against the tip of a probe. This application of force to a probe simulates the probing process and provides an indication of how the probe will function in the probe test cell. Force load cells in the mechanism that support a conductive post measure the relative force exerted by the probe against the conductive post as a function of the magnitude of the deflection to determine how the probe behaves mechanically. Finally, the small size of the conductive post allows the probe tip position to be verified by noting the position of the tip of the conductive post (by means of encoders associated with the support mechanism of the conductive probe) when the conductive post contacts the probe tip and makes an electrical connection. The nominal XY position of a probe may be specified a priori or determined optically.
Problems can arise however, where only a single probe is overtraveled. Overtraveling a single probe of an entire array or of a sub-set of a probe array can introduce differential deformation. Differential deformation, in the context of probe inspection, pertains to introducing undesirable variation into the mechanical characteristics of a probe with respect to other similarly situated probes of a probe card. In some instances that could manifest as an undesirable modification of the free hanging position of a probe tip due to some change in the probe itself or in the mechanism, structure or materials that secure the probe to the probe card. Differential deformation may also result in changes in the mechanical characteristics of a probe such as where fatigue in the probe results in a change in the spring constant of the probe. In other instances the electrical characteristics of a probe may change as where damage to a probe reduces conductivity or where deformation of the probe results in short circuiting of the probe. These problems apply primarily to cantilever-type probes, but can also apply to other types of probe card probe types such as blade, buckling beam, vertical, array, multi-DUT, micro-spring, etc. These examples are illustrative only and are not intended to be an exhaustive list of issues that may arise.
Further, it is desirable to avoid overtraveling probes too many times. Each probe of a probe card array has a more or less fixed life and can only withstand so much wear on the probe tips and only so much mechanical stress before a failure may occur. While this is clearly a statistical determination, in general, the fewer the number of times a probe is overtraveled during testing and analysis, the better.
Accordingly, there is a need for a probe card analysis mechanism and a related method that reduces the amount of differential deflection that may be experienced by the probes of a probe card and which also limits the number of times that a probe is overtraveled. Further, there is a need for such a mechanism and/or method to be flexible enough to be readily adaptable to different types of probe cards and/or to different probe array geometries or requirements.